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Post: Swift struct type recursion

Swift struct type recursion

Published 12:06 Jun 11, 2018.

Created by @ezra. Categorized in #Programming, and tagged as #iOS.

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the enums in Swift support recursive types using the indirect keyword so you can do something like:

indirect enum Tree<T> {
    case node(left: Tree?, right: Tree?, element: T)

and the classes in Swift also support recursive types:

class TreeNode<E> {
  var leftNode:TreeNode<E>
  var rightNode:TreeNode<E>
  var element:E

It's feels like that the ability to declare a binary tree node as a struct with recursive types in it to be the most natural implementation:

struct TreeNode<E>{
  var leftNode:TreeNode<E>
  var rightNode:TreeNode<E>
  var element:E

But you just can't do it with structs:

Value type 'TreeNode<E>' cannot have a stored property that recursively contains it.

Why can't structs have recursive value types in Swift? Is this a temporary limit of the language or is it as intended?

The answer is in the question: structs are value types. If you include a substruct B into a struct A, it means, that one object of type A will have a size sizeof(all_other_fields_of_A) + sizeof(B). So, a value type can not be recursive: it would have infinite size.

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